Well, I encourage you to fill out and define this aspect of the world as we go along, but generally...
The most common mainstream religion - dominant in this region, prominent everywhere - is outwardly fairly familiar to us. It is built around worship of one, all-powerful God, with a powerful if disunited church. It’s all rather more dualistic than Christianity, though. Against God of land, light and warmth is a silent and nameless god of the sea, dark and cold. They have battled for all eternity. The withdrawal of the seas is sure sign of the true God’s triumph - but all live in fear of the tides turning and the endless black sea reclaiming its own. Still, it’s a pretty optimistic faith - God is constantly granting the faithful new land and new frontiers, after all.
You can see how easily the Mer can be denounced as the spawn of the sea and worshippers of the deep. Some actually do worship the Nameless, mostly humans; a dualistic faith easily lends itself to such heresies. After all, who is to say the new land isn’t the gift of the sea to Man? Who better to ensure survival in the Rime than the god of dark and cold? These cults tend to be frowned upon in polite society. In the interior, the churches persecute them as the heresies they are.
The faith generally holds that in the beginning, God brought down the first land to stand above the hungry sea. Of all living creatures, there were two. Slowly, gradually, the land began to expand. Life expanded with it, and on and on this process goes until the present. Scientific experiments certainly support this idea, at least partially - the existence or non-existence of one singular island at the beginning and where it is now is a heated topic of debate.
It’s not that big of a deal to be an atheist or non-religious, though. The power of the churches has been on the decline for a good century and city folk in particular are fairly secular.